By Chris Hickling
A Fire & Gas System (FGS) philosophy provides a solid foundation for the design of an effective gas detection system, which in turn helps protect plant and personnel from gas releases and resulting flammable and/or toxic effects. An FGS philosophy for a process facility that is not fit for purpose or does not have a firm auditable basis can increase the likelihood of undetected leaks incurring risk to personnel or unnecessary expenses for the company. Under-engineering a gas detection system has safety implications, while over-engineering has commercial implications such as increasing capital and maintenance costs without significantly reducing risk.
The workflow for an FGS philosophy can be summarized in the following steps:
Assess FGS requirements – review regulatory requirements, corporate standards, pertinent Process Hazard Analysis (PHA) recommendations, and Recognized And Generally Accepted Good Engineering Practices (RAGAGEP)
Develop FGS philosophy and procedures – review materials and properties (flammable, toxic, or inert), process flow, and risk tolerance criteria
Define FGS scenarios and zones – determine hazards using data for process conditions, weather, occupancy, and airflow data, and drawings such as plot plans, Piping & Instrumentation Diagrams (P&IDs), and Cause & Effects (C&Es)
Define zone FGS performance requirements – Develop criteria to assess facility layout and define areas
Develop criteria for FGS detector placement
So what makes an effective FGS philosophy?
Firstly, it's important to establish the scope of the FGS system. Is it intended to protect on-site personnel and equipment, offsite community, and/or environment? A comprehensive review of the entire facility is essential – if individual process units at the facility are exclusively analyzed for gas detection, it could result in a fractured response and unforeseen effects at other units. A gas cloud doesn't care where it's released or where it's going, and it doesn't respect boundaries.
Secondly, the FGS philosophy should follow applicable codes like NFPA 72 and be applied consistently across the facility. Issues could arise if different areas of the plant use different gas detection technologies or alarm levels; for example, if one unit of the plant alarms at 10% of the Lower Explosive Limit (LEL) and another unit alarms at 20% LEL, or there are inconsistent color of warning strobes for a toxic gas release. Consistency in gas detection, alarms, and encompassing standardized procedure(s) helps the operators and employees respond efficiently.
Additionally, the FGS philosophy should lay down the criteria for decisions on gas detection required and appropriate mitigative response such as alarm levels (e.g., alarm at 10% LEL). The FGS philosophy also helps decide the voting criteria for the number of gas detectors to take action. For example, two out of two (2oo2) gas detectors may be required to alarm before starting the sprinkler system or dumping Halon.
Finally, an FGS philosophy should be auditable. During its development, assumptions are made which feed into how the gas release is modeled and location of gas detectors. If a bad assumption is made and a leak later occurs, it is essential to be able to revisit the original FGS philosophy and assess the original basis for the design. If the gas detection system has a performance-based design, the layout of the system is documented so that it can be reviewed and adjusted. Traditional rules of thumb gas detector placement do not offer this ability to review and update the basis.
Once these practices are incorporated into a facility's FGS philosophy, a comprehensive and well-documented FGS philosophy provides a solid foundation for the design of an effective and auditable gas detection system. Facilities can have a dependable basis to ensure an appropriate number of gas detectors in the appropriate locations, potentially lowering risk and minimizing costs for the gas detection system. nfpa gas detection